The 2019-2020 tropical cyclone season for Australia has officially come to an end. The season played out generally as expected when predictions were made back in September and October of 2019, with a slow start to the year, followed by a burst of frequent systems. The finish to the season was quieter than expected, but this wasn’t a surprise in the moment given how the tropics played out. Above image via RAMMB / CIRA of Severe TC Damien.
Back in September / early October 2019, we forecast a total of 10 tropical cyclones for Australia, 5 of those becoming severe (Category 3+) with 4 systems in WA, 2 in the NT and 4 in QLD.The reality was close to that with 8 firm systems occurring (9 loose, as UESI was technically NOT classed as a cyclone despite producing Category 2 strength winds over Lord Howe Island) – 3 of those were severe with 5 systems in WA, 1 in the NT and 3 in QLD (with UESI being a loose 4th). We had 5 landfalls with the reality being 3. An additional 15 tropical lows were forecast that wouldn’t become tropical cyclones, in reality there ended up being 16. We consider this to be a fairly accurate prediction.
Summary map of the 2019-2020 Australian Cyclone Season. Image via wikipedia.
Lets break down the season. It was a slow start to the 2019-2020 tropical cyclone season as a strongly positive Indian Ocean Dipole and a warmer than normal ENSO (warm side of neutral) delayed the start of the monsoon across the Tropics as the monsoon trough remained over Indonesia and even North of the equator for an extensive period of time. Models were hesitant on a tropical cyclone occurring prior to Christmas (we normally only have 1 maybe 2 anyway prior to Christmas) and this was the case. Models were also very keen on conditions rapidly changing after New Year. On January 4, Cyclone Blake was named in the Indian Ocean and on January 4, a low pressure system over the Timor Sea would eventually become Severe TC Claudia. Blake remained a Category 1 and made landfall over the Western Kimberley while Claudia hightailed it into the Indian Ocean, narrowly missing the Kimberley and intensifying into a Category 3. Claudia ended up being responsible for producing over 500mm in 24hrs for some locations SW of Darwin and broke the NT daily rainfall all-time record. There was then a lull in activity before a new monsoon wave occurred at the start of February and this helped spawn Severe Tropical Cyclone Damien and Cyclone Uesi. Damien intensified quickly and made landfall at Dampier / Karratha as a strong Category 3 system which lead to some damage in the area along with flooding. Uesi on the other hand, bypassed the Pacifc Islands of Vanuatu and New Caledonia as a Category 3 system before weakening to officially become a subtropical cyclone (cyclone without a warm core) in Australian waters where it still maintained Category 2 strength. Esther then formed in the Gulf of Carpentaria 2 weeks later and went on a slow moving rampage over the Gulf and Northern NT into the Northern WA region before back tracking across the NT and into SW QLD and NSW where record-breaking rainfall was recorded. After Esther, things got quiet quickly with Ferdinand becoming a Category 3 system in the Indian Ocean at the end of February, followed by TC Gretel in the Coral Sea during mid March, Cyclone Harold at the start of April was a weak system in Australian waters before racing into the Pacific Island group as a high-end Category 5 monster, and then Cyclone Jeruto moved in from Africa into Australian waters mid April – all of these systems posed no threat to Australia.
Severe TC Damien making landfall over the Pilbara Coast on February 8th. Image via Weatherzone
The fall out from the season was the Northern Tropics of Queensland has seen an extremely dry Summer with most areas around 30-50% below average for rainfall. The Northern NT areas are also below average by about 15-20% despite having an extremely active period during mid January to early March (outside of this timeframe its been extremely dry) and its about 15% below average for rainfall across the Northern WA areas. Parts of the Pilbara and North West NT have seen localised phenomenal rainfall due to systems directly impacting these areas, but this is on an extremely localised scale. While Inland QLD has seen no systems track down the middle which was of high hope pre-season. The only area of QLD that really benefited was the Far South West which saw 2 days of rainfall with Esther that shattered records. The benefit of a quiet season has been minimal damage and minimal loss of life directly from these systems.
Australian rainfall anomalies for the TC Season (October 1 to April 30) via BOM.
Summary of the 2019-2020 Australian Cyclone SeasonThomas2020-05-02T12:10:36+10:00
Today seems like the perfect time to explain why its snowing in TAS and why its hot in QLD at the same time. This in fact is a very common occurrence for both to happen simultaneously and thats because its the same weather system that causes both.
The general overview is we have a large high pressure system over Southern WA and across the Great Australian Bight. This is combining with a very strong and powerful low pressure system down near Antarctica that is feeding a low pressure trough up across Coastal parts of Eastern Australia and across the tropics. What this does, is draw moisture in over South Eastern AUS which includes Southern NSW, VIC, TAS and South East / Southern Central SA as indicated by the arrows on image 1. The trough along the East Coast of Australia however turns these winds more West to South West and after travelling over land for a good 12-18 hours those winds begin to dry out. The positioning of the trough along the Coast though prevents any moisture coming in off the Coral Sea and this results in showers occurring over South East AUS, but dry conditions across Northern NSW, much of QLD and through Interior parts of the Country.
Surface winds over AUS with isobars showing South Westerly winds dominating much of the Country due to a trough and high pressure region. Image via Windy
Then we look in the 850mb range for temperatures (image 2) and that explains a whole lot more. A strong upper trough (very strong for this time of year) is active over the Eastern Bight, TAS and VIC (indicated by the blue colouring which is temperatures below 0ºc at 850mb / 2km above the ground). This cold air is mixing with the moisture to produce snowfall over SE AUS. At the same elevation, temperatures are into the low to mid 20’s across the Northern half of AUS.
850mb temperatures (about 2km above the ground) showing very cold air over SE AUS and very warm to hot air over Northern AUS. Image via Windy.
So now… Queensland, Northern Territory and Northern Western Australia are seeing dry winds at the surface, and hot air aloft – this ALWAYS produces hot / above average temperatures. But on the other hand.. Tasmania, SE South Australia, Victoria and Southern NSW have very cold air aloft and moisture which = snow.
This setup is very typical of a Winter setup with upper troughs racing across Southern Australia and a lack of rainfall over Northern Australia. The only difference now compared to then, is the snowfall becomes somewhat limited (10-15cm expected instead of 50cm) and the temperatures over Northern AUS are much hotter because it is November and NOT Winter.
Maximums across AUS for Friday, November 8th via OCF/BSCH showing cold over SE AUS and hot over Northern AUS.
Why is QLD HOT when TAS is COLD, and visa versa??Thomas2019-11-07T12:13:12+10:00
22 degree halo’s, or what they are more commonly described as… sun / moon halo, are much more common than people realise and also spark interest when they appear. 22 degree halo’s are optical phenomenons which is derived from the family of ice crystal halos. They form a ring around both the sun and moon at a radius of approximately 22º from either one. While its occurrence with the sun is typically just called a sun halo, the moon version can also be called a winter halo (as well as a moon halo) due to the more likely time for it to occur at night is during winter.
22 degree halo’s typically form from the sun or moon refracting light into millions of minuscule, tiny hexagonal shaped crystals floating in the upper atmosphere. While they generally will only ever occur on fine days or clear nights, the presence of some cloud cover (mostly cirrostratus cloud) can help indicate the presence of a halo. Even if the sky appears completely clear, cirrostratus cloud can be so fine and thin that the simple art of seeing a halo resembles the presence of the cloud.
Sun halo captured over the Sunshine Coast by Jeff Higgins
Despite it being one of the most common types of halo (occurring up to 100 times a year), the ironic thing is, its a circular halo shape which is created and solely responsible by hexagonal ice crystals. The exact shape of these ice crystals is still debatable, even to this day, but scientists for now believe they are most likely hexagonal and the prism shape of them is what helps bend the light. The other conclusion which is possible is bullet-shaped clusters of ice crystals… but the exact shape isn’t really necessary for us at this stage, the end process is that its still light refracting through ice crystals.
While the conditions typically occur during calm, peaceful conditions… the visualisation of the phenomenon, especially with moon halos is linked to the approach of worsening weather over the next few days. In saying that, it is loosely linked and somewhat unreliable.
Moon halo captured over Newcastle in 2015 by Rochelle Buckton
Altostratus clouds are some of the most dull and boring clouds across the sky. This type of cloud is quite layered and featureless, with a grey colouring. The Latin words album and stratus mean spread out or flattened with height.. which is exactly what these clouds are, medium to high flat and dull clouds.
Altostratus clouds are very large, mid level thin sheets and are usually composed of a mixture of water droplets and ice crystals. They typically don’t produce precipitation. They are thin enough that the sun can seep through them, but they often retain a grey colouring with no identifiable structural features about them.
They form when cirrostratus clouds descend from much higher altitudes in the sky, and while theres nothing special about these clouds they can help produce optical phenomenon’s such as iridescence and coronas. Due to their featureless appearance, there are no categories that this cloud fits into, it essentially is its own cloud. There are some different patterns though that may occur and these include undulatus, radiatus and duplicatus.
Altocumulus clouds often form ahead of frontal systems, mostly occluded fronts but sometimes cold fronts. As the front passes through, altostratus layers deepens and bulk out to become nimbostratus clouds which can produce rain or snow. This can typically mean that the visual sighting of these clouds refers to a change in the weather over the next 12-24 hours.
What are Altostratus Clouds?Thomas2019-07-08T02:22:08+10:00
Altocumulus clouds are small clouds situated in the mid levels of the atmosphere, between roughly 7,000 and 18,000ft. These small clouds are called cloudlets, which are mostly shaped like rounded clumps. Altocumulus clouds are made up of a combination of both ice and water which allows them to have a slightly ethereal appearance compared to those of the cumulus variety which are slightly bigger in size and fluffier in appearance.
Altocumulus clouds can form through several different methods. In all of these methods, they produce a key visual difference which allows them to be differentiated from cirrocumulus clouds. That key visual is the presence of shading, as the clouds appear to retain a ‘greyer’ colouring around their base. This is largely due to their elevation, where the cirrocumulus clouds are higher in the atmosphere and the sun is rarely “above” them, but also shining light below them… the altocumulus clouds may be lower than the sun in comparison to the horizon and thus the sun’s light doesn’t project across the entirety of the cloud. They can form through the break up of altostratus clouds. They can also form through the lifting process of moist air pockets which become cooled by mountainous terrain, this process can produce atmospheric waves which can produce the clouds.
While the weather is generally fairly tame when altocumulus clouds are present, they can on the off occasion produce precipitation. This precipitation won’t touch the ground though as its evaporated before it touches the ground, this precipitation is called virga.
Altocumulus clouds are categorised into 4 different types and while their mechanics are similar they all have their own unique characteristics which help determine which is which. These categories are Stratiformis, Lenticularis, Castellanus, Floccus.
Altocumulus Stratiformis: These are the most common types of altocumulus cloud. They are flat-bottomed puffy clouds, packed tightly together but separated by small rivers of sky. These can sometimes cover the entire sky.
Altocumulus Lenticularis: These are arguably the most spectacular cloud, out of all the cloud types possible. Altocumulus Lenticularis clouds, also known as lenticular clouds, are lens-shaped clouds that form over hilly or mountainous terrain and are often referred to as ‘UFO’ clouds or ‘spaceship’ clouds due to their appearance.
Altocumulus Castellanus: These are great indicators of instability in a localised area, as Altocumulus Castellanus towers can often lead to the formation of cumulonimbus clouds or thunderstorms. These are much taller clouds than they are wide, but they retain a puffy appearance.
Altocumulus Floccus: These are often spotted alongside altocumulus castellanus clouds, however they are slightly smaller and more ragged cloudlets. These are often found with virga present also.
Altocumulus stratiformis via Frank Le Blanq
What are Altocumulus Clouds?Thomas2019-07-08T02:17:04+10:00
Cirrostratus clouds are high altitude clouds that are mostly transparent. They cover large areas of the sky which can sometimes lead to the formation of sun or moon halos (also known as a 22 degree halo). The appearance of a 22 degree halo may be the only indication that the clouds are even present due to how transparent and thin they become. The latin words cirrus and stratus mean lock or tuft and flattened / spread out which is exactly what these clouds look like – spread out locks of hair covering the sky.
Cirrostratus clouds typically sit no lower than 20,000ft in the atmosphere and can reach as high as 40,000ft. They from as a result of slowly rising air. They are usually generated at the forefront of frontal weather systems such as warm fronts and cold fronts and the movement of the clouds can be used to predict the weather over the next 24 hours. Cirrostratus clouds are also closely linked to vapour contrails as planes fly through the dry atmosphere.
While cirrostratus clouds don’t typically produce any weather themselves, they are fantastic indicators of approaching weather. There are 2 different forms or categories of cirrostratus clouds. (1) Cirrostratus Nebulous which likely indicates that an incoming front will likely produce persistent rain within 24 hours or (2) Cirrostratus Fibratus which likely indicates that stratus clouds may proceed it and produce precipitation (most likely in the form of light drizzle or rain). Cirrostratus clouds can span for thousands of kilometres and due to their transparent appearance, the sun is able to shine through them but at the same time, they can be slightly dense enough to still cast shadows on the Earth’s surface.
Cirrostratus Clouds with the sun reflecting through via Stephen Burt
What are Cirrostratus Clouds?Thomas2019-07-07T21:36:27+10:00
If you thought altostratus clouds were dull and boring, wait for nimbostratus clouds. These are considered the most dull and boring of the cloud types. Nimbostratus clouds are essentially extensive dark or grey, gloomy, featureless layers of thick cloud that block out the sun and produce persistent rain. There is nothing picturesque about them at all, the only thing one could argue is that altostratus clouds are more dull because they don’t produce precipitation whereas nimbostratus at least produces rain.
These clouds typically sit quite low in the atmosphere, around the 2,000-10,000ft mark. The lower they are, the more likely they are to produce heavier rainfall, however rainfall or precipitation (whether its snow, rain or any other form) is likely regardless. These clouds by definition are mid-level clouds as they predominantly sit closer to the 10,000ft however as stated above, they can fall to as low as 2,000ft which is matching it with some of the lowest clouds in the atmosphere.
Nimbostratus clouds form through the deepening and thickening of an altostratus cloud and is often associated with frontal systems, similarly to the altostratus cloud. Nimbostratus clouds will often bring precipitation that can last for several hours – whether its rainfall or snowfall, until the associated passes over. The only precipitation this cloud doesn’t produce is hail. If hail is present then by definition it becomes a cumulonimbus cloud.
Similarly to the altostratus cloud, nimbostratus clouds aren’t categorised into seperate groups like most of the other clouds.
What are Nimbostratus Clouds?Thomas2019-07-07T21:27:35+10:00
Stratocumulus clouds are the most common cloud on planet Earth. They are essential low level, clumps or patches of cloud which vary in colour from bright white to dark and dull grey. These common clouds are recognised with their defined bases, some of which are darker than others. They usually have gaps between to signify their individuality but its not uncommon for them to become joined together.
Stratocumulus clouds typically form from status clouds breaking up. They are indicators of a change in weather and are usually present near any type of frontal system (cold, warm or occluded). While these clouds are typically fairly dull on the weather front, they can produce showery conditions. These unsettled conditions however are usually due to other clouds being present with the stratocumulus clouds, not the stratocumulus clouds themselves. In reality, they rarely produce anything more than drizzle.
Stratocumulus clouds are categories into 4 seperate groups, while the mechanics that make up stratocumulus clouds are the same for each group.. their are slight differences that help determine which is which. These include Stratiformis, Cumulogenitus, Lenticularis and Castellanus.
Stratocumulus lenticularis via jim Galvin
Stratocumulus Stratiformis: These are the most common of the common and are basically flat based layers of cloud with a few cracks between each individual cloud.
Stratocumulus Cumulogenitus: These typically occur when cumulus clouds encounter a temperature inversion which suppresses their development and growth from transitioning between cumulus and cumulonimbus. These then spread outwards and clump together.
Stratocumulus Castellanus: These are a much thicker and much more drizzly type of stratocumulus cloud. Turreted tops form when convection initiates through the stable layer and allows the stratocumulus to extend upwards. This can potentially lead to the formation of cumulus congestus or even cumulonimbus thunderstorm clouds.
Stratocumulus Lenticularis: These are the rarest of the stratocumulus family and are often spotted around hilly or mountainous terrain. They have a very different appearance to the far more spectacular lenticular cloud. They form when hills or mountains produce atmospheric waves which then contribute to their lens-shaped appearance.
Stratocumulus Castellanus via Barry Chan
What are Stratocumulus Clouds?Thomas2019-07-07T21:21:00+10:00
These are the lowest clouds on average across the entire globe, ranging anywhere from 1,200ft to the surface (0ft). These are quite layered clouds and are fairly uniformed grey or white colour. They are often last long periods of time and are often the scene of dull and overcast days. When they near or reach the surface (0ft) they are what we commonly consider fog or mist.
Stratus clouds form in calm and very stable conditions where gentle breezes raise cool and moist air over the colder land or ocean surfaces. These clouds can exist in a variety thicknesses and are sometimes opaque enough to darken the day and allow for as little light as possible to come through.
Stratus clouds are usually accompanied by next to no rainfall, however if they are thick enough they can produce light drizzle. This drizzle can also fall in the form of snow if its cold enough.
Stratus clouds are only defined by two different categories. The first being stratus nebulous which is a featureless dark layer, capable of producing drizzle. The second is stratus fractus, this is a straight layer which is starting to break up or dissipate and this is where the breaks in cloud being to appear.
Stratus Undulats – Not a very common type of stratus cloud that has some gaps allowing light to penetrate through, as well as some undulations in the cloud formation. Image via Martin Gudd
What are Stratus Clouds?Thomas2019-07-07T21:01:05+10:00
After 5 days of relentless well above average heat, the strongest heatwave for November in over 2 decades for SEQLD, the cool change is FINALLY on its way! Above image showing the change circled in blue, with the cooler South and South West winds marked in purple via BSCH.
Since Saturday until today (Wednesday), large parts of Inland QLD and the South East corner of the State have been under a relentless heatwave. A heatwave that has seen several locations break 20+ year old streaks for November standards along with some November records being challenged and broken. While the Coastal strip in SEQLD wasn’t so bad, it had the added influence of high humidity… making the days just as uncomfortable as those areas further Inland that saw the very high temperatures.
Some of the streaks and records that were achieved include. *Gatton recorded 3 straight days above 39ºc and 4 straight days above 37ºc+, the first time for both in November since 1996 (22 years) *Toowoomba recorded back to back 35ºc+ days for the first time in November since 1996 (22 years) *Ipswich recorded 4 straight 36ºc+ days for the first time in November since 1996 (22 years) *Warwick recorded 4 straight 35ºc+ days for the first time in November since 2009 (9 years) *Miles, Stanthorpe, Dalby, Oakey, Kingaroy, Gympie, all recorded their hottest November streaks since 2014 (4 years) *Ballera recorded 5 straight days above 41ºc for the first time in November since 2012 (6 years)
From a records point of view, *Ipswich recorded its hottest November night on record with 23.8ºc (records dating back to 1941) *Charleville 29.4ºc & St George 28.6ºc had their warmest nights since 1965 for November, the second warmest night ever for Charleville in November (0.2ºc short of the 1965 record) *Windorah recorded a very warm 31.7ºc night *Birdsville recorded a 31.1ºc night *Several towns recorded 28ºc+ nights across Southern Inland, Central Inland, North West, South West and Western QLD at some stage, with any other nights during the heat period being 25ºc+
The change is already being seen through South West QLD where today Birdsville maxed at just 22.1ºc and Ballera 22.4ºc – a massive 19ºc colder than yesterday! The current temperature in Ballera is a mere 15.7ºc, thats 22ºc colder than this time yesterday as well.
Forecast maximums cross Southern and SE QLD tomorrow (Thursday) via BSCH / OCF
While temperatures will remain near average tomorrow (Thursday) across South East QLD, the humidity will be taken way out of the equation so it will feel very comfortable. Tonight will remain fairly warm due to cloud coverage trapping some of the heat in, but the cool change should still move through and bring with it much drier South to South Westerly winds which will cut the humidity out of the air and make it actually feel quite pleasant. Across Inland areas it should be much cooler tonight, with temperatures dropping into the low teens. This should transition into a much colder day tomorrow (something similar to what SW QLD saw today) (see image above).
Forecast minimums via OCF / BSCH for Wednesday night into Thursday morning
Here comes the cool change!Thomas2018-11-07T19:36:58+10:00